close coupling

In atomic physics, close coupling is a quantum mechanics method to calculate the multi-electronic atomic and molecular structure from fine structure to hyperfine structure levels and dynamic processes including photoionization,{{Cite journal|last=Lu|first=K. T.|date=1971-08-01|title=Spectroscopy and Collision Theory. The Xe Absorption Spectrum|journal=Physical Review A|volume=4|issue=2|pages=579–596|doi=10.1103/physreva.4.579|bibcode=1971PhRvA...4..579L |issn=0556-2791}}{{Cite journal|last=Seaton|first=M J|date=1983-02-01|title=Quantum defect theory|journal=Reports on Progress in Physics|volume=46|issue=2|pages=167–257|doi=10.1088/0034-4885/46/2/002|s2cid=250797547 |issn=0034-4885}} collisional excitation and ionization as well as autoionization and their inverse processes. In this method, the multi-electron systems are treated as a loosely interacting electron with a target ionic or neutral atomic as well as molecular, in which the electrons are strongly interactive with each other. The interactive atomic or molecular complex system is reduced into a so-called (N+1) problem. Based on this scheme, the inter-channel interaction, that is, configuration interactions (CI) are involved.

Integrated with other techniques, especially the matrix techniques and multi-channel quantum defect theory, close-coupling method could provide precise structural and dynamical studies of atomic and molecular systems.